  We discuss how variations in the scanning tunneling microscope (STM) tip,
whether unintentional or intentional, can lead to changes in topographic images
and dI/dV spectra. We consider the possibility of utilizing functionalized tips
in order to improve the sensitivity of STM experiments to local irregularities
at the surface or hidden below the surface layers. The change in the tip
symmetry can radically alter the contrast of the topographic image due to
changes in tip-surface overlap. The dI/dV curves change their shape according
to which sample bands the tip orbital tends to overlap. In addition, relative
phases between competing tunneling channels can be inverted by changing the tip
symmetry, which could help reveal the origin of a local irregularity in
tunneling spectrum.
