Axing artefacts for accurate atomic force microscopy

Carbon nanotubes (CNTs) could soon replace silicon as the material of choice for the probe tips used in atomic force microscopy (AFM), report US and Korean nanotechnologists.

AFM measures the forces between an oscillating tip and a sample as the tip travels across the sample, producing an image of that sample’s surface. Tips have traditionally been made of silicon, but CNT tips are now being investigated as they offer several advantages. CNT tips tend to last longer than silicon tips. They are also thinner, so are better able to investigate the nooks and crannies of tiny nanostructures. And they are stiffer, so are better at imaging soft surfaces, such as biomolecules.

Unfortunately, CNT tips can also generate certain false images, termed divots and ringing artefacts. The researchers, led by Arvind Raman from Purdue University, West Lafayette, have now discovered what causes these false images and suggested ways to avoid them.

Divots in an image occur when the CNT tip scans down over the side of a structure, such as a trench, while ringing artefacts occur when scanning at the bottom of a high structure, such as the floor of a trench. The researchers studied the interaction of the tip and the surface and found that both image artefacts were caused by the thin CNT tip temporarily sticking to the surface at these points.

It should be possible to prevent this happening. ’You can avoid getting these artefacts if you know how to set the parameters,’ said team member Mark Strus from Purdue University. ’For example, you can change your set point or your amplitude and still get a good image with your nanotube.’ Jon Evans