First Principles Study of the Effect of Tip Shape on Scanning Tunneling Microscopy Images

Effects of the shape of probe tip on observed scanning tunneling microscopy images have been investigated by first-principles calculations by taking the Si(111)√3×√3-Ag surface as an example of sample. By using three tungsten clusters with different shapes and by tilting their symmetry axes, various shapes of the tip were represented. Even for those tips which produce images distinctly different from experimental ones when their symmetry axes are normal to the surface, the images change so as to agree with experiments as the axes are tilted gradually. The results indicate that only one atom at the tip apex usually makes an essential contribution to the tunneling current and can explain why an experimental tip prepared quite arbitrarily usually produces good images.