Colloidal Synthesis of Mn2+-Doped SrTiO3 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.