Hydrothermal Synthesis and Optical Properties of Cr-Doped Bi₂Se₃ Topological Insulator Nanoplatelets

Incorporation of magnetic Cr dopants into the Bi₂Se₃ topological insulator (TI) system has predominantly been achieved in bulk single crystals and thin films, whereas Cr doping at the nanoparticle level has not been realized. Cr-doped Bi₂Se₃ thin nanoplatelets were successfully synthesized via a hydrothermal route, establishing a wet-chemical approach to producing magnetically doped TI nanoparticles. Structural and quantitative energy dispersive X-ray spectroscopy compositional analyses reveal substitutional incorporation of Cr³⁺ into the Bi₂Se₃ lattice up to 4.8 ± 0.4 at. %, independent of the nominal precursor concentration, thereby defining the solid solubility limit of Cr in Bi₂Se₃ nanoparticles prepared under hydrothermal conditions. Excess Cr is accommodated through the formation of a Cr-rich secondary phase. UV-vis absorption spectroscopy demonstrates enhanced optical absorption across the 200-1000 nm spectral range upon Cr incorporation. In contrast, cathodoluminescence (CL) spectroscopy on individual Cr-doped nanoplatelets shows that despite Cr doping, the CL emission remains localized at nanoplatelet edges and exhibits spectral characteristics identical to those of pristine Bi₂Se₃. These results demonstrate that hydrothermal synthesis is a viable route for preparing Cr-doped Bi₂Se₃ nanoplatelets and provide insights into dopant incorporation limits and their impact on nanoscale optical properties.