Peak Force Atomic Force Microscope is a new technique to characterize fragile materials such as nanoparticles with high accuracy with only one measurement. Unlike the tapping mode AFM, Peak Force AFM operates at a frequency below the resonant frequency of the cantilever. This allows for a direct control of the forces and avoids lateral forces that may damage the sample as in contact mode AFM. Furthermore, the performance characteristics of Peak Force AFM are suitable to work also in Tunneling AFM (TUNA) mode, enabling the study of the electrical properties of materials.
In this work SnS nanoparticles capped with tri-n-octylphosphine oxide (TOPO) have been characterized. By means of Peak Force AFM it is possible to measure simultaneously topography and current maps of nanoparticles, yielding information about the shape, size and the conductivity of even a single nanoparticle. The topography map clearly showed single nanoparticles with a size less than 5 nm and spherical shape. In the conductivity map it is possible to discern the same nanoparticles, the correlation with the topography map is evident. This confirms the conduction (though not calibrated) of SnS nanoparticles. This type of measurements has been repeated many times in order to check the reproducibility of this technique. Moreover, the same nanoparticles have been measured also by Torsional Resonant TUNA AFM in order to compare it with Peak Force AFM. By means of TR-TUNA it was possible to measure the topography of SnS nanoparticles capped with TOPO but not the current. Besides, the resolution of the topography map acquired by TR-TUNA AFM is inferior to Peak Force AFM. From this comparison it has been found that the conductivity of nanoparticles, even if they are capped with TOPO, can be measured by Peak Force AFM, a result that thus far has been difficult to achieve by other types of AFM.