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

Abstract

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 figureof-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.

Dopant Selection Strategy for High Quality Factor LSPR from Doped Metal Oxide Nanocrystals

“…The LSPR of undoped In2O3 NCs is attributed to the accumulation of electron density due to oxygen vacancies whereas in Zr:In2O3 NCs, Zr doping is largely responsible for compensating the free electrons. The highly symmetric shapes of the LSPR peaks in Figure 3 indicates there is little ionized impurity scattering, 16,[21][22]27 consistent with our initial hypothesis. The LSPR parameters of different Zr:In2O3 NCs are listed in Table ST1.…”

“…The LSPR parameters of different Zr:In2O3 NCs are listed in Table ST1. Quantitatively, the LSPR FWHM of 735 cm -1 (91 meV) for 1.3% Zr:In2O3 NCs is very close to that reported for Ce-doped In2O3 NCs (77 meV), 26 In-doped CdO (67 meV) 18 and smaller than the lowest reported LSPR FWHM for Sn:In2O3 (99 meV) 22 and other doped metal oxide NCs (Table ST2); hence Zr:In2O3 NCs exhibit one of the narrowest LSPR peaks yet reported. In terms of LSPR Q-factor, Zr:In2O3 NCs display rather high values over a range of LSPR energies that are almost identical to those of Ce-doped In2O3 NCs and subsequently, one of the highest in the literature.…”

“…However, similar trends have routinely been reported in doped metal oxides and, in particular in Sn:In2O3, where the lattice expansion is attributed to the repulsion between the Sn 4+ ions which cannot be compensated entirely by the electron density in the lattice. 22,[47][48] We suggest that a similar mechanism is underlying the trend reported here with Zr 4+ ions incorporated in the In2O3 lattice across the range of compositions we synthesized, with competing factors influencing the lattice constant. Our understanding that Zr is well incorporated in the lattice is further substantiated by the systematic increase in the FWHM of the XRD peaks with an increase in Zr doping signifying a systematic decrease in the crystallite size with Zr doping.…”

“…For instance, Sn 4+ is a shallow donor in In2O3, donating electrons to the conduction band of In2O3 and increasing the LSPR energy. [20][21][22] However, apart from acting as ionized donor impurities, Sn dopants in In2O3 also hybridize with In 5s orbitals, leading to significant renormalization of the band curvature at the conduction band minimum, [23][24][25] thereby changing the effective mass, increasing the LSPR FWHM, and decreasing the electron mobility. 16 Cerium is an alternative dopant for In2O3 or a co-dopant with Sn that has been reported to minimize ionized impurity scattering.…”

“…53 Surface segregated doping can be favorable as previous work has shown it is associated with narrow LSPR and high Q-factors. [21][22]54 Dopants segregated near the surface allow charge carriers to move inside the lightly doped or undoped NC core without scattering from ionized impurities. The reduced scattering minimizes LSPR damping, thereby decreasing the LSPR FWHM and enhancing electron mobility.…”