Imaging and direct manipulation of nanoscale three-dimensional features using the noncontact atomic force microscope

We report on a remarkable image contrast reversal in noncontact atomic force microscope (NC-AFM) imaging of nanosized three-dimensional (3D) particles. We show that the image contrast of such 3D particles can switch from positive to negative as a function of NC-AFM imaging conditions and this occurs during, both, in situ ultrahigh vacuum imaging and imaging in air. Our results indicate that the contrast reversal can arise from a tip-sample interaction force-gradient-dependent instability of the NC-AFM feedback loop. Exploiting the above instability to induce selective tip-sample contact, we propose and demonstrate a protocol for the controlled nanomanipulation of 5 nm diameter gold particles, in air at room temperature using the NC-AFM. The contrast reversal phenomenon is proposed to be universal, suggesting its potential applicability to nanomanipulation in a variety of materials systems.