Noncontact Atomic Force Microscopy - Yale School of Engineering ...

Spatial force fields above a single atom defect
André Schirmeisen 1 and Domenique Weiner 2
1 CeNTech (Center for Nanotechnology) and Institute of Physics, University of Muenster, Germany
2 SPECS Zurich GmbH, Switzerland
Tu-1140
Atomic force microscopy under ultrahigh vacuum conditions is a powerful tool to
investigate the atomic structure of surfaces. The method of 3D force field spectroscopy
[1] allows the spatial analysis of vertical and lateral interatomic forces [2], as well as the
potential energy landscape with atomic resolution [3]. In this study we focus on the
analysis of surface defects on a NaCl(001) crystal by 3D force field spectroscopy. The
spatial force fields along different crystallographic directions were measured above a
defect, which appeared as a valley of molecular dimensions in the surface topography.
We find that the vertical tip-sample force directly above the defect is repulsive. From the
force fields we calculate the atomic scale potential energy landscape, which is compared
to model calculations. This model is based on electrostatic interactions of hard spheres
and assumes an ion terminated tip apex. According to this model our experimental
potential energy fields agree best with a situation where a single ion is missing in the
surface. This raises interesting questions about the unexpected stability of single charge
defects in an ionic surface.
Figure 1: Left: Topography scan of NaCl(001) showing a surface defect. Right: Frequency shift
slice extracted from 3D spectroscopy data along a row of ions including the surface defect. The
force and energy profiles best agree with a model assuming a singular missing ion.
[1] Hölscher, Langkat, Schwarz, Wiesendanger, Appl. Phys. Lett. 81, 4428 (2002)
[2] Ruschmeier, Schirmeisen, Hoffmann, Phys. Rev. Lett 101, 156102 (2008)
[3] Schirmeisen, Weiner, Fuchs, Phys. Rev. Lett. 97, 136101 (2006)
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