Study of Plasmonic and Magneto-Optical Properties of Transition Metal Doped Indium Oxide Nanocrystals

Abstract

Plasmonic nanostructure materials have been widely investigated recently because of their considerable potential for applications in biological and chemical sensors, nano-optical devices and photothermal therapy. Compared to metal nanocrystals (NCs), doped semiconductor NCs with tunable localized surface plasmon resonance (LSPR) from near-infrared (NIR) mid-infrared (MIR) region bring more opportunities to the applications of plasmonics. Magnetoplasmonic nanostructures which could be utilized in multifunctional devices also have attracted attention due to the combination of plasmonic and magnetic properties and the manipulation of light with external magnetic fields. In this research, indium oxide (In2O3) as a typical n-type semiconductor with high mobility and carrier concentration is selected as the host lattice for doping, and molybdenum (Mo) and tungsten (W) which are transition metal elements from the same group as dopants. Colloidal molybdenum-doped indium oxide (IMO) NCs and tungsten-doped indium oxide (IWO) NCs with varying doping concentrations have been successfully synthesized, and their plasmonic and magneto-optical properties have been explored. Similarities and differences between IMO NCs and IWO NCs were discussed. Both IMO and IWO NCs have shown good tunability of plasmon resonance in the MIR range approximately from 0.22 eV to 0.34 eV. 9.2 % IMO NCs show the strongest LSPR at 0.34 eV and the maximum free electron concentration of 1.1×1020 cm-3, and 1.5 % IWO NCs exhibit the strongest LSPR at 0.33 eV with the free electron concentration of 0.94×1020 cm-3. The magneto-optical properties were studied by magnetic circular dichroism (MCD) spectroscopy. The variable-temperature-variable-field MCD spectra that coincide with the band gap absorption, indicate the excitonic splitting in the NCs. A robust MCD intensity at room temperature suggests intrinsic plasmon-exciton coupling and carrier polarization induced by plasmon, which might be phonon-mediated. A decrease in MCD signal with temperature and the saturation-like field dependence of MCD intensity for IMO and IWO NCs may related to the different oxidation states of the dopant ions since the reduced 5+ oxidation states can exhibit the Curie-type paramagnetism. IMO and IWO NCs show the coupling between exciton and plasmon in a single-phase which opens a possibility for their application in electronics and photonics. Moreover, magnetoplasmonic modes provide a new degree of freedom for controlling carrier polarization at room temperature in practical photonic, optoelectronic and quantum-information processing devices.