  Isotope effects on heat conduction and convection have been known for
decades. However, whether thermal radiation can be isotopically engineered
remains an open question. Here, we predict over 3-orders-of-magnitude variation
of radiative heat flow with varying isotopic compositions for polar dielectrics
at room temperature. We reveal this as an isotope mass effect which induce
phonon line shift and broadening that in turn affect phonon-mediated resonant
absorption both in the near and far field. In contrast, the isotope effect is
negligible for metals and doped semiconductors which largely depend on free
carriers. We also discuss the role of temperature with regard to surface mode
excitation.
