Size-Induced Enhancement of Carrier Density, LSPR Quality Factor, and Carrier Mobility in Cr–Sn Doped In₂O₃ Nanocrystals

Heterovalent dopant ions, such as Sn 4+, in In 2 O 3 nanocrystals (NCs) provide free electrons for localized surface plasmon resonance (LSPR). But the same heterovalent dopants act as electron scattering centers, both independently and by forming complexes with interstitial oxygen, thereby increasing LSPR line width. Also, such complexes decrease free carrier density. These detrimental effects diminish the figure-of-merit of LSPR known as the quality factor (Q-factor). Herein, we designed colloidal Cr–Sn codoped In 2 O 3 NCs, where both high carrier density and low carrier scattering can be achieved simultaneously, yielding a high LSPR Q-factor of 7.2, which is a record high number compared to prior reports of doped In 2 O 3 NCs. Q-factors increase systematically from 3.2 for 6.6% Sn doped In 2 O 3 NCs to 7.2 for 23.8% Cr–6.6% Sn codoped In 2 O 3 NCs by increasing the Cr codoping concentration, which is also accompanied by an increase in NC size from 6.7 to 22.1 nm. Detailed characterization and analysis of LSPR spectra using Drude model suggest that the increase in NC size (induced by Cr codoping) is mainly responsible for the enhanced LSPR Q-factor. Sn 4+ dopants on the surface of NCs are more vulnerable to form irreducible complexes with interstitial oxide ions, compared to Sn 4+ ions in the core. Therefore, an increase in the concentration ratio of [Sn core ]/[Sn surface ] (or [Sn]/[interstitial oxide]) by increasing the size of NCs, increases the carrier density. Furthermore, such increase in both NC size and Cr doping influences multiple factors reducing the scattering of charge carriers, thereby increasing the optical carrier mobility. This unique combination, which increases both the density and mobility of charge carriers, improves the LSPR Q-factor.