  The diversities in crystal structures and ways of doping result in extremely
diversified phase diagrams for iron-based superconductors. With angle-resolved
photoemission spectroscopy (ARPES), we have systematically studied the effects
of chemical substitution on the electronic structure of various series of
iron-based superconductors. In addition to the control of Fermi surface
topology by heterovalent doping, we found two more extraordinary effects of
doping: 1. the site and band dependencies of quasiparticle scattering; and more
importantly 2. the ubiquitous and significant bandwidth-control by both
isovalent and heterovalent dopants in the iron-anion layer. Moreover, we found
that the bandwidth-control could be achieved by either applying the chemical
pressure or doping electrons, but not by doping holes. Together with other
findings provided here, these results complete the microscopic picture of the
electronic effects of dopants, which facilitates a unified understanding of the
diversified phase diagrams and resolutions to many open issues of various
iron-based superconductors.
