Plasmonics and Magnetism in M (Mn, Cr)-Sn Codoped In2O3 Colloidal Nanocrystals

Abstract

Recently, localized surface plasmon resonance (LSPR) has been observed in doped semiconductor nanocrystals (NCs), with distinct differences compared to LSPR from metallic NCs. In our Sn4+-doped In2O3 NCs, commonly known as ITO NCs, Sn4+ doping provides free electron in the conduction band, which in turn gives rise to LSPR. Controlling the dopant concentration allows to control carrier density, which in turn can tune the LSPR peak from near-to-mid infrared region without changing the size and shape of NCs. In addition to Sn4+ doping, we attempted to codope the system with magnetic ions such as Cr3+ and Mn3+, where concentration of each dopants can be varied in the range 0-10%, and size of NCs are in the range of 7-11 nm with possibility to observe magneto-plasmonic, and magneto-electric effects in future. Mn3+ precursors were used to substitute isovalent In3+ ions from the lattice of In2O3 NCs. However, after careful characterization involving powder x-ray diffraction, and Q-band EPR, the product NCs were found to be doped with Mn2+ ion, both in the presence and absence of Sn4+ codopant which suggest the conversion is happening by accumulating electrons either from oxygen vacancies. Influence of this reduction is observed in LSPR data, where both absorbance and energy of LSPR peak systematically decreases with increasing Mn content in the NCs, due to decrease in electron density which can be rationalized by electron hole recombination. Since Mn doping decreases carrier density, we attempted to dope Cr3+ instead of Mn2+. This is because prior electronic structure calculations shows that the unoccupied d-levels of Cr3+ are energetically higher than the conduction band minimum of In2O3, and therefore, electrons will not get trapped in the Cr3+ d-levels. If so, then the LSPR of Cr-Sn codoped In2O3 NCs should not shift with Cr3+ doping, and to our surprise, LSPR shifts toward higher energy with Cr3+ doping. Such shift towards higher energy is opposite to all other codoped systems such as Fe-Sn and Mn-Sn codoped NCs. Four probe electrical conductivity data shows increase in conductivity by increasing Cr3+ concentration from 1 to 10% for Cr-Sn coped In2O3 NCs. These electrical and plasmonic data suggest that the Cr3+ doping does not trap free carriers as predicted by prior theory, but the exact reason of the LSPR shifting towards higher energy is not yet clear.