  We present first-principles total-energy electronic-structure calculations
that provide the microscopic mechanism of the Ag atom diffusion between the
half unit cells (HUCs) on the Si(111)-(7x7) surface with and without the tip of
the atomic force microscope (AFM). We find that, without the presence of the
AFM tip, the diffusions between the two HUCs are almost symmetric with the
energy barrier of about 1 eV in the both directions. The diffusion is a
two-step process with an intermediate metastable configuration in which the Ag
atom is at the boundary of the HUCs. With the presence of the tip, we find that
the reaction pathways are essentially the same, but the energy barrier in one
direction is substantially reduced to be 0.2 - 0.4 eV by the tip, while that of
the diffusion in the reverse direction remains larger than 1 eV. The Si tip
reduces the energy barrier more than the Pt tip due to the flexibility of the
tip apex structure. In addition to the reduction of the barrier, the tip traps
the diffusing adatom preventing the diffusion in the reverse direction. Also we
find that the shape of the tip apex structure is important for the trapping
ability of the adatom. When the tip apex structure is blunt, the adatom
interacts with the tip atom other than the tip apex atom. The bond formation
between the AFM tip atom and the surface adatom is essential for the atom
manipulation using the AFM tip. Our results show that the atom manipulation is
possible with both the metallic and semiconducting AFM tips.
