Colloidal Synthesis of Mn ²⁺ -Doped SrTiO ₃ Nanocrystals: B-Site Substitution and Spin-Relaxation Dynamics

We report the colloidal hydrothermal synthesis of a unique nanocrystal (NC) system where Mn2+ ions are introduced into SrTiO3 and appear to substitute at the octahedral Ti4+ site instead of the A site that has been reported from solid-state syntheses. This observation is supported by electron paramagnetic spectroscopy, where the Mn2+ signal has a hyperfine splitting value that is more consistent with Mn2+ at the B site in bulk SrTiO3. We also do not detect any higher-valent Mn3+ or Mn4+ by any spectroscopic method in the Mn:SrTiO3 NCs. The spin-relaxation dynamics of the Mn2+ dopants before and after introducing Ti3+ defects into the SrTiO3 NCs through photodoping is also studied qualitatively using continuous-wave EPR power saturation studies at 100 K. We observe broadening of the Mn2+ EPR line widths consistent with cross-relaxation between Ti3+ and Mn2+ up to room temperature. This result is similar to but less efficient than our previous demonstration of accelerated spin relaxation of Cr3+ after photodoping Cr3+:SrTiO3 NCs. This weaker cross-relaxation results from the larger difference in Landé g factors that results in a larger energy mismatch between the Mn2+ and Ti3+ EPR transitions in an applied magnetic field.