Tip Characterization from AFM Images of Nanometric Spherical Particles

Since atomic force microscopy (AFM) images are a composite of probe and sample geometry, accurate size determinationsareproblematic. A relatively straightforwardmathematical procedure for determining tip radius of curvature (RT) for an asymmetrical tip was recently developed by Garcia et al. (ProbeMicrosc. 1997, 1, 107). This study represents an experimental test of that procedure for both silica (∼150 nm) and polystyrene (∼50nm)nanospheres. Theprocedure canbe summarizedby twosteps: (1) tip characterization assuming that the observed AFM height is a true measure of a spherical particle’s diameter and (2) use of the tip shape to extract a calculated width. To ensure that AFM heights were equivalent to the true width, a direct comparison of individual particle sizes determined by transmission electron microscopy (TEM)andAFMwas conducted. Heightsmeasured fromAFMimages of polystyrenenanospheres differed, on average, less than 5% fromwidthsmeasured by TEM. The quality ofRT valueswas therefore evaluated by the magnitude of relative error in calculated particle widths with respect to true widths. For the tip used in this study a calculated RT of 13 nm resulted in excellent calculated widths for both polystyrene and silica spheres. While spherical particles whose diameter is less than RT (such as 5-nm Au colloids) can be used to characterize the tip apex, larger diameter spheres are required to fully characterize the tip. However, spheres much larger thanRT predominantly interact with the walls of the tip and therefore yield artificially high RT values. On the basis of our analysis of the procedure developed by Garcia et al., the best sphere size for full characterization of the tip (apex and walls) is one in which both portions of the tip interact with the sphere to similar extents (approximately: RT eRp e 2RT, whereRp is the particle radius).