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

P.I-09
Contact potential difference on the atomic-scale probed by
Kelvin Probe Force Microscopy: an imaging scenario
Laurent Nony 1 , Adam Foster 2 , Franck Bocquet 1 , and Christian Loppacher 1
1
Aix-Marseille Université, IM2NP, Av. Escadrille Normandie-Niemen, F-13397 Marseille and CNRS, IM2NP (UMR
6242), Marseille-Toulon, France
2
Department of Physics, Tampere University of Technology,
P.O. Box 692 FIN-33101 Tampere, Finland
A fully numerical analysis of the origin of the atomic-scale contrast in Kelvin probe force
microscopy (KPFM) is presented. The numerical implementation mimics recent experimental
results on the (001) surface of a bulk alkali halide single crystal for which a simultaneous
topographical and Kelvin, so-called Contact Potential Difference (CPD), atomic-scale contrast
had been reported [1]. In this work, we have combined atomistic simulations of the tip-sample
force field with our non-contact AFM/KPFM simulator [2] to compute topographical and CPD
images. The force field notably includes the required short-range bias voltage dependence to
account for the CPD atomic-scale contrast. The sample is a NaCl(001) single crystal. The
atomistic simulations have been performed by means of the code SCIFI [3]. The simulator has
been used in the Frequency-Modulation KPFM operating mode. The tip consists of a spherical
metallic cap within which a 4x4x4 NaCl cluster with the [111] direction pointing towards the
surface is partly embedded. The sample consists of a 10x10x4 NaCl slab embedded within a
semi-infinite continuous medium merely described by its dielectric constant. The sample, with a
macroscopic height (several millimeters), lies on a metallic counter-electrode that is biased with
respect to the tip. Beyond the short-range chemical and electrostatic forces computed with SCIFI,
we have included a usual Van der Waals long-range term and a less usual, although fundamental,
long-range electrostatic interaction. This term mimics the weak capacitive interaction between tip
and sample holders and is necessary to describe the behavior of the CPD with the distance.
Figs.1a and b show that the simulator is able to account for the simultaneous topographical and
CPD atomic-scale contrast, respectively. The occurrence of such a contrast is intricately
connected to the dynamic polarization of the ions of the slab triggered by the modulation of the
bias voltage, unless otherwise no Kelvin contrast is measured. The nc-AFM/KPFM simulator is a
tool which helps in interpreting the experimental data quantitatively.
References :
[1]- F. Bocquet et al., Phys. Rev. B
78, 035410 (2008)
[2]- L. Kantorovich et al., Surf. Sci.
445, 283 (2000)
[3]- L. Nony et al., Phys. Rev. B 74,
235439 (2006); L. Nony et al.,
accepted in Nanotechnology
Fig.1a- Topography channel
measured at 0.45nm to the
surface: the vertical contrast is
40pm
100
Fig.1b- simultaneously
acquired CPD channel: The
vertical contrast is 0.6V