Vertical and Lateral Manipulation of adatoms on surfaces

In this work, we use total energy calculations based on interaction potentials from the Embedded Atom Method to calculate the effect of a simulated AFM tip on the diffusion barriers for adatom manipulation. We study the effect of the tip on lateral as well as vertical manipulation. The systems considered are different metal surfaces with varying complexities and with an adatom, and the tips are either sharp or blunt with a (100) or a (111) geometry.

The Embedded Atom Method :

During lateral manipulation, it was found that a sharp (100) tip was most effective in reducing the barrier but the blunt (100) tip lowered the barrier for a larger range of lateral separations between the adatom and tip apex. Ag was found to have the lowest barrier for the adatom to diffuse towards the tip.

Some tabulated results and figures:    

Model system for lateral manipulation
	Metal Barrier in absence of tip (meV) Barrier in presence of tip (meV) Opbarrier in presence of tip (meV) Shift in saddle point (angstroms) Ag 215.5 50.1 283.6 0.7 Cu 267 61.7 366.5 0.6 Ni 308.3 131.3 386.4 0.45 Pd 355 190.1 426.9 0.4 Au 416.7 324.3 414.9 0.12 Pt 620.8 462.5 606.2 0.12
Click on the image for a larger version	Tabulated data for several metals - Lateral Manipulation
3D plot of the Potential Energy Surface	Contour plot of the Potential Energy Surface

Vertical manipulation calculations using a (100) blunt tip on flat, stepped and kinked surfaces, revealed that it is easiest to manipulate an adatom on a stepped surface as compared to a flat or a kinked surfaces.

Some tabulated results and figures: