Reduction of Mn³⁺ to Mn²⁺ and near infrared plasmonics from Mn–Sn codoped In₂O₃ nanocrystals

Abstract

Colloidal Sn doped In₂O₃ nanocrystals have gained considerable attention for exhibiting surface plasmon resonance (SPR) which can be easily tuned in the near to mid infrared region by controlling the dopant concentration. Codoping these NCs with magnetic ions such as Fe²⁺, Mn^2+/3+ can help develop interactions between delocalized electrons and localized magnetic spins which are required for spin-based applications. We prepared colloidal Mn–Sn codoped In₂O₃ nanocrystals with a diameter of ∼6–7 nm, starting from a Mn³⁺ precursor for Mn doping. Detailed characterization including Q-band electron paramagnetic resonance spectroscopy show that Mn exists in the 2+ oxidation state in both Mn-doped and Mn–Sn codoped In₂O₃ nanocrystals, in spite of using the Mn³⁺ precursor. This aliovalent Mn²⁺ doping along with charge neutralization through oxygen vacancies are energetically more favorable when compared to isovalent Mn³⁺ substitution in the In₂O₃ lattice. The SPR band decreased both in intensity and energy with increasing Mn content in the Mn–Sn codoped In₂O₃ nanocrystals. This is because Mn²⁺-doping introduces a hole in the lattice promoting electron–hole recombination reducing the free electron density. Also, Mn²⁺ dopants scatter the electrons thereby broadening the SPR band.