Origin of atomic resolution on metal surfaces in scanning tunneling microscopy

Scanning tunneling microscopy has repeatedly resolved individual atoms on a number of metal surfaces with atomic distances 2.53. This is in sharp contradiction to the resolution limits previously predicted, 69. We present a theory of such atomic resolution in terms of actual tip states, for example, dz2 tip states on tungsten tips. Quantitative interpretation of the observed images is obtained with no adjustable parameters. We predict that to achieve atomic resolution, the tip material should be either a d-band metal or certain semiconductor. © 1990 The American Physical Society.