Atomic Force Microscopy (AFM) is widely used for nanoscale characterization of materials by scientists worldwide. The long held believe of AFM, is that the tip is generally chemically inert but can be functionalized in respect to the studied sample. This implies that basic imaging and scanning procedures do not affect surface and bulk chemistry of the studied sample. However, an in-depth study of the confined chemical processes taking place at the tip surface junction, and the associated chemical changes to the material surface have as of now been missing.
Here, we used a hybrid system that combines Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) with an AFM to investigate the chemical interactions that take place at the tip-surface junction. Investigations showed that even basic contact mode AFM scanning is able to modify the surface of the studied sample. In particular, we found that AFM tips are heavily contaminated by the silicone oils, which were determined to come from standard gel-boxes used for the storage and handling of the tips. These oils were deposited from the AFM tip into the scanned regions during the scanning process, which modifies surface state of the studied sample and can significantly affect results of the surface investigations performed by the AFM. Furthermore, we found that silicone oils can be spread to distances exceeding 15 microns from the static tip.
These results demonstrate that chemical phenomena have to be taken into the consideration for interpreting and understanding results of AFM mechanical and electrical studies relying on state of the tip-surface junction.
This work was conducted at the Center for Nanophase Materials Sciences, which is a Department of Energy (DOE) Office of Science User Facility.