title: Nano-contact microscopy of supracrystals.
abstract: BACKGROUND
  Highly ordered three-dimensional colloidal crystals (supracrystals) comprised of 7.4 nm diameter Au nanocrystals (with a 5% size dispersion) have been imaged and analysed using a combination of scanning tunnelling microscopy and dynamic force microscopy.
  
  
  RESULTS
  By exploring the evolution of both the force and tunnel current with respect to tip-sample separation, we arrive at the surprising finding that single nanocrystal resolution is readily obtained in tunnelling microscopy images acquired more than 1 nm into the repulsive (i.e., positive force) regime of the probe-nanocrystal interaction potential. Constant height force microscopy has been used to map tip-sample interactions in this regime, revealing inhomogeneities which arise from the convolution of the tip structure with the ligand distribution at the nanocrystal surface.
  
  
  CONCLUSIONS
  Our combined STM-AFM measurements show that the contrast mechanism underpinning high resolution imaging of nanoparticle supracrystals involves a form of nanoscale contact imaging, rather than the through-vacuum tunnelling which underpins traditional tunnelling microscopy and spectroscopy.
authors.affiliation: The School of Physics and Astronomy, The University of Nottingham, Nottingham, NG7 2RD, U.K.
authors.forename: Adam
authors.initials: A
authors.lastname: Sweetman
authors.affiliation: Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, Monaris, F-75005, Paris, France ; CNRS, UMR 8233, Monaris, F-75005, Paris, France.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Goubet
authors.affiliation: The School of Physics and Astronomy, The University of Nottingham, Nottingham, NG7 2RD, U.K.
authors.forename: Ioannis
authors.initials: I
authors.lastname: Lekkas
authors.affiliation: Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, Monaris, F-75005, Paris, France ; CNRS, UMR 8233, Monaris, F-75005, Paris, France ; CEA/IRAMIS, CEA Saclay, 91191, Gif-sur-Yvette, France.
authors.forename: Marie Paule
authors.initials: MP
authors.lastname: Pileni
authors.affiliation: The School of Physics and Astronomy, The University of Nottingham, Nottingham, NG7 2RD, U.K.
authors.forename: Philip
authors.initials: P
authors.lastname: Moriarty
keywords: dynamic force microscopy; nanoparticle; non-contact atomic force microscopy; point contact imaging; scanning probe microscopy; supracrystal