Dopant Selection Strategy for High Quality Factor LSPR from Doped Metal Oxide Nanocrystals
Thin films of degenerately doped metal oxides such as those of Sn-doped In2O3 (Sn:In2O3) are commercially significant for their broad utilization as transparent conducting electrodes in optoelectronic devices. Over the last decade, nanocrystals (NCs) of Sn:In2O3 and other doped metal oxides have also attracted interest for localized surface plasmon resonance (LSPR) that occurs in the near to mid-infrared region. The suitability of this LSPR for some applications depends on its capacity to concentrate light in small regions of space, known as near-field hot spots. This efficiency to create near-field hot spots can be judged through an LSPR figure-of-merit such as Quality factor, defined as the ratio of LSPR peak energy to its linewidth. The free electron density determines the LSPR peak energy while the extent of electron scattering controls the LSPR linewidth and hence these factors together essentially dictate the value of the Quality factor. An unfortunate tradeoff arises when dopants are introduced since the aliovalent dopants generating the free electrons (increasing LSPR energy) also act as centers of scattering of electrons (increasing LSPR linewidth), thereby decreasing the LSPR Quality factor. Dopant selection is hence of paramount importance to achieve a high value of LSPR Quality factor. Here, we describe the properties of aliovalent cationic dopants that allow both high LSPR energy and low LSPR linewidth and, subsequently, high LSPR Quality factor. In this context, we identify Zr4+ as a model aliovalent dopant for high LSPR Quality factor in the In2O3 lattice. The resulting Zr-doped In2O3 NCs exhibit one of the highest LSPR Quality factors reported in the mid-infrared region while also performing equivalently to the recognized materials for either high dopant activation (Sn:In2O3 NCs) or low LSPR linewidth (Ce-doped In2O3 NCs), simultaneously. The Zr donor level is positioned well into the conduction band of In2O3 and Zr doping is surface segregated, both minimizing electron scattering. The combination of this low electron scattering and high dopant activation of Zr4+ ions is responsible for the high LSPR Quality factors. These strategies can be used to design a variety of doped metal oxide NC systems exhibiting high LSPR Quality factors.