Dopant Selection Strategy for High-Quality Factor Localized Surface Plasmon Resonance 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 past 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 (Q-factor), defined as the ratio of LSPR peak energy to its line width. The free electron density determines the LSPR peak energy, while the extent of electron scattering controls the LSPR line width; hence, these factors together essentially dictate the value of the Q-factor. An unfortunate trade-off 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 line width), thereby decreasing the LSPR Q-factor. Dopant selection is hence of paramount importance to achieve a high value of LSPR Q-factor. Here, we describe the properties of aliovalent cationic dopants that allow both high LSPR energy and low LSPR line width and, subsequently, high LSPR Q-factor. In this context, we identify Zr⁴⁺ as a model aliovalent dopant for high LSPR Q-factor in the In₂O₃ lattice. The resulting Zr-doped In₂O₃ NCs exhibit one of the highest LSPR Q-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 line width (Ce-doped In₂O₃ NCs) simultaneously. The Zr donor level is positioned well inside 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 Q-factors. These strategies can be used to design a variety of doped metal oxide NC systems exhibiting high LSPR Q-factors.