Highly ordered three-dimensional colloidal crystals (supracrystals) of gold nanoparticles have been imaged and analysed using a combination of scanning tunnelling microscopy and dynamic force microscopy. By exploring the evolution of both force and tunnel current with respect to tip-sample separation we make the surprising _nding that single nanoparticle resolution is readily obtained in tunnelling microscopy images acquired more than 1 nm into the repulsive (i.e. positive force) regime of the probe-nanoparticle interaction potential. This implies that the contrast mechanism involves a form of \point contact » imaging, rather than the through-vacuum tunneling, which underpins traditional tunneling microscopy and spectroscopy. Constant height force microscopy has been used to map tip-sample interactions in this point contact regime, revealing inhomogeneities that arise from the convolution of the tip structure with the ligand distribution at the nanoparticle surface.
Source : Point-contact probe microscopy of nanoparticle supracrystals. A. Sweetman, N. Goubet, I. Lekkas, M. P. Pileni, and Philis Moriarty J. Nanotechnology, 2014 , ID 6647414