The simulation of non-contact atomic force microscopy (NC-AFM) has evolved considerably during the last decade. A number of significant successes have been achieved in understanding the mechanism of image contrast on various surfaces, such as CaF2 and TiO2 . The methods of image simulation have almost always been carried out within a static picture, with the surface responding adiabatically to the microscope tip, which moves extremely slowly compared to phonon frequencies. The recently developed virtual atomic force microscope (VAFM) [1] is a software package that explicitly models the electronics and feedback loops of a real NC-AFM. In conjunction with Kinetic Monte Carlo or master equation formulations [2] of thermal transitions at the surface, the use of the VAFM allows us to go beyond the static formulation and to investigate dynamical processes that occur at the surface in real time.
The first system we consider is a Ca defect on a MgO (001) surface. We find that the defect caused a surprisingly strong perturbation on the microscope tip. We investigate the conditions for imaging a defect that exerts a significant short range force on the tip, which can lead to instabilities and/or tip crash before the electronics can respond.
Ab initio calculations suggest that adsorbed water molecules on Ceria (111) surfaces can rotate at a rate similar to that of the tip oscillations. This leads to the image being averaged over several orientations of the water molecule during scanning. We investigate the broadening in the image caused by the molecular rotation, and the effect of changing temperature thereby “freezing” the water, relative to the tip oscillations.
A final system is Pd atoms adsorbed on MgO (001). Here we have fully mapped the potential energy surface for the motion of a Pd atom as a function of the tip height [3]. We then investigate the conditions necessary to achieve stable imaging of the adsorbate, and show that controlled manipulation of the metal atom should also be achievable using NC-AFM.
[1] Polesel-Maris J, Gauthier S, J. App. Phys., 97, 044902 (2005)
[2] Trevethan T, Watkins M, Kantorovich LN, et al., Phys. Rev. Lett. 98 028101 (2007)
[3] Trevethan T, Watkins M, Kantorovich LN, et al., Nanotechnology 17, 5866 (2006)
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