Tip-sample interaction in the AFM characterization of bio-plant nanostructures

The feasibility of using well know 3D nanostructures as tip characterizers is today an issue for critical dimensions measurements at the nanoscale by means of Atomic Force Microscopy (AFM). Among other, an accurate correction of the tip shape and size is needed to reduce the uncertainty of AFM-measurements of linewidth standards, sidewalls and non-spherical nanoparticles. Our study aims at investigating the Tobacco Mosaic Virus (TMV) as a tip characterizer. Samples were prepared on cleaved mica by depositing a few drops of purified and diluted suspension of TMV in water and let them to dry in air. Serial dilutions of the purification were prepared in water and observed, in order to reach a concentration in which viral particles were abundant but isolated and not overlapping. The rod-shaped TMV has a diameter of about 18 nm obtained from XRD measurements, and represents a reference in size and shape at the nanoscale. AFM characterizations of TMV particles have been reported in the last decade and earlier. The diameter of the TMV is mostly determined as the top height of the rod from the reconstructed profile of isolated virions on the AFM image. Nevertheless, due to the tip-sample-substrate interactions the AFM-reconstructed top-height of the rod is smaller than the reference value. These interactions are discussed with reference to experimental data and models in literature in order to determine the elastic deformations and associated uncertainty of corrections, which largely reduce the difference of the AFM-reconstructed top-height diameter from the reference value. An uncertainty budget is provided for the tip-shape correction calculated by lateral and top-height diameter of the TMV rod. As a remark, a quite good stability of the TMV samples deposited on mica was observed by repeated observations over time.